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Dec 1, 2008 12:00 AM
Spectrophotometers play a key role in quality assessment. Printers use them to check their production processes and customers use them to evaluate print jobs. Often, the spectrophotometer reading determines a job's acceptability. But when VIGC, the Flemish Innovation Center for Graphic Communications in Turnhout, Belgium, did a study on device accuracy, it found deviations up to a delta E of nearly 4.
“Color quality is the biggest challenge in the printing industry,” says Eddy Hagen, VIGC's managing director. “Graphic arts companies will try everything to meet customers' color expectations. For many customers, color is the most important criterion to accept or reject a print job. So you would expect that [color measurement device] quality is top class. But it isn't.”
After experiencing some issues with different devices, VIGC started its first tests to compare multiple devices in the summer of 2007. “We saw some deviations between the different spectrophotometers that we use ourselves,” explains Fons Put, senior consultant with VIGC. “So we set up a procedure to check and compare different devices. We used the GretagMacBeth NetProfiler test chart as the reference. It comes with a certificate stating the L*a*b* values of the different patches, measured with three spectrophotometers under ideal conditions. The certificate is valid for 12 months — it should be renewed every year. And then we measured the 13 patches on the test chart with different spectrophotometers. For two patches, we also measured devices' repeatability [by taking]10 measurements in a row.”
Over the past year, VIGC has tested more than 20 devices, most of which are used by printing companies it works with. Other studies used devices the researchers got directly from the vendors. VIGC tested devices that are out in the field, that are used on a daily basis by actual printers.
When a customer demands a maximum delta E of 2 — often the case for quality print jobs in Belgium — he or she wants a device that measures the color as accurately as possible. However, the VIGC study revealed deviations up to delta E = 3.77 for specific colors. On average, the deviation per instrument of all 13 patches is 1.56.
VIGC encountered multiple devices of the same type or the same brand. Is there a relation between the type, the brand and the accuracy? “That's an interesting question,” says Put. “There was one general rule: The newer devices perform better. With devices that were a few years old, sometimes we got good results with the first one and bad results with the second one. Our own main spectrophotometer, which is calibrated regularly on that NetProfiler chart, was the best of them all. Another device, the same brand, the same type, more or less the same age, performed poorly.”
When the measurements of all 13 patches were averaged per device, VIGC found deviations from the exact value ranging between delta E 0.45 for the best device and 2.74 for the worst one. Several devices showed — on average — higher deviations than the margins customers expect from their printers for high quality print jobs. The highest deviation for individual patches was a delta E of 3.77. “Also interesting — or disturbing, if you like — was that one brand had quite strong deviations in the red and orange,” reports Put. “We found this on multiple devices of that specific brand.”
Even within a certain type of device, VIGC found considerable differences. The graph above shows the deviations from the absolute value for seven devices of the same brand and type.
With older devices, poor maintenance is one of the culprits. “We know some devices performed poorly because the optics or the calibration tile were dirty,” says Put. Spectrophotometers need regular calibration and periodic cleaning. Light source is another factor. “No light source has a perfect ‘spectral power distribution,’” says Put. “And if you don't have much power in certain wavelengths, not much color can be reflected in that region, which limits the accuracy of detecting small variations in that color region. An LED light source has a completely different spectral power distribution from a gas filled tungsten bulb. And both are used in spectrophotometers.”
The big differences that were found can cause trouble: Customers demand a delta E of 2, but their measurement device might be for a delta E of 3. According to the researchers, a simple solution for the industry would be to accept delta E 2000 as the formula to calculate color differences.
“When people talk about delta E, they usually refer to delta E*ab, also known as delta E 1976. This is also the formula that is mentioned in the relevant ISO standards. But this formula is very inaccurate when it comes to small color differences,” says Hagen. “I can show you a pair of colors with a difference of delta E 5 which is barely noticeable. Take a 100 percent and a 95 percent process yellow from ISO coated. The deviation is just noticeable, but if you calculate it with delta E*ab, you get a figure of 5. Delta E*ab doesn't really conform to the human perception of color differences. The newer delta E 2000 does. Take the same yellow color pair and you will get a delta E of approximately 1, which conforms to the initial idea of delta E: A delta E of 1 is the smallest noticeable color difference.”
When the test results of VIGC are recalculated with the newer delta E 2000, the figures become much more realistic. The overall average of all devices on the 13 patches is a rather bad 1.56 when delta E*ab is used, but a very good 0.39 when calculated with the more recent delta E 2000.
Hagen continues, “The bizarre thing is that some experts don't want to use delta E 2000 because it is not that good when it comes to large color deviations. In those cases, the old delta E*ab performs better. But who is interested in the accuracy of large color deviations? I want accuracy in small color deviations. That is where the battlefield is, where print jobs get rejected. Not because the colors look very different, but because the delta E formula states that they are different. The printing industry would benefit if the delta E 2000 formula would be the official formula for calculating color differences. But all relevant ISO standards only seem to know delta E*ab. Even the draft for the upcoming update of ISO 13655 on color measurement only talks about delta E*ab. Which [helps neither] the printing industry nor its customers.”
Summarizing their findings, the researchers said the measurements from a spectrophotometer are not absolute. Variation can exist between different devices. Devices need to be calibrated on a regular basis and properly maintained. Periodic vendor cleaning might seem expensive, but it should be weighed vs. the cost of a perfect print job that is rejected due to a drifting spectrophotometer.
VIGC urged the industry and the standard organizations to consider using delta E 2000 as the standard to calculate color differences when judging print quality. For small color differences, delta E 2000 conforms much better to human vision than delta E*ab. “Rejecting jobs because of color differences should be about seeing differences, not just about measuring a certain number,” says Put.
Color is a core competency for VIGC (Vlaams Innocatiecentrum voor Grafische Communicatie, Flemish Innovation Center for Graphic Communication). VIGC does the compliancy testing of preflight applications for the Ghent PDF Workgroup. E-mail managing director Eddy Hagen at email@example.com.
GATF's Joe Marin concurs that poor housekeeping can contribute to inaccurate color measurement. “Devices that reside in the pressroom obviously are much more susceptible to the accumulation of debris, particularly spray powder and paper dust,” he says. “I've found that of the printers who have spectrophotometers, few calibrate them regularly and even fewer realize that they need to be cleaned on a regular basis.”
Marin notes backing type also can impact measurements. “Is it a black surface under the press sheet, or white, or gray, or just another press sheet? The standard calls for a black surface for proper measurement.”
“The study points out some valid problems when measuring color and appearance,” says Larry Goldberg, technical director for Beta Industries (Carlstadt, NJ). “All of the following points come into play, and determining which ones have the strongest effect is an important part of the solution.”
“A spectrophotometer is a very sensitive and precise instrument,” says George Adam of TECHKON USA (Danvers, MA). “The wide variance in measurement between instruments is usually magnified by not calibrating the device often and by not recertifying the device at the factory after two years of usage.”
Some manufacturers offer a utility that lets the user check the accuracy of the instrument on a daily or weekly basis. TECHKON's ColorCHECKER, for example, includes ceramic tiles for checking the device's curent state vs. the technical specification. Ceramic tiles are the standard for calibrating instruments in the factory.
“Even two new spectro-photometers will read colors differently because of their internal circuitry and optics,” says Adam. “The ISO has made tremendous progress over the last few years in outlining the baseline specifications for the design and manufacture spectrophotometers. Following these guidelines will ensure closer inter-instrument agreement.”
Adam agrees with VIGC's sugges-tions about delta E 2000. “[It] is more accurate than the original delta E formula, which was intro-duced in 1976. Delta E 2000 is more sensitive to neutrals, flesh tone and lighter colors, and it is easier for human eyes to detect smaller differ-ences in these colors. Darker and saturated colors tend to be harder for the human eye to differentiate.”